Review of ocean tidal, wave and thermal energy technologies
Introduction
Sun provides more than 99.99% of energy and earth contributes about 0.01% [1]. Fossil fuels are a form of antediluvian eon solar energy. All sources of energies, except geothermal and nuclear, are ultimately powered by the sun [2]. Earth radiates heat and its thermal energy come from radioactive decay (80%) and planetary accretion (20%) [3]. Oceans encompass over 70% of the earth's mass. Ocean tides are caused by earth's gravitational interaction with the moon (68%) and sun (32%). The impact of the moon is 2.6 times more than the sun due to its shorter distance from earth. Ocean waves are caused by friction of winds with the water surface. Earth has lost 17% of its rotational energy due to its slow deceleration rate of 12.19µs/y [4]. Oceans are a great form of renewable energy which is stored in the form of thermal energy (heat), kinetic energy (tides and waves), chemical energy (chemicals) and biological energies (biomass). Tidal current or wave generators harvest kinetic energies, and osmotic power plants and thermo-electric generators reap salinity and thermal gradients [5]. Up to date ocean power technologies and barriers are reported in the literature [6], [7].
Climate change, S curved growths, population, energy and power crises require to explore and harvest renewable energy resources. Akin to hydropower, wind and solar energies the ocean dynamism is an ideal energy resource. The tide is a periodic rise and fall of water in the seas and oceans, twice during a lunar day (24Hr and 50 min). Ocean water remains at the maximum level for 50 min at different times on different days repeating the cycle every 19 years. Sea and ocean levels at different locations depend on their latitudes and shore. Lunar motion around the earth increases time interval between successive tides from 12 h to 12 h and 25 min. The earth, moon and sun become aligned every two weeks, at new and full moon days, to create maximum height spring tides. On waxing and waning half moon days, in the first and third quarter, the sun being at 90° creates least height neap tides. The performance of tidal and wave power plants depends on days of months and wind speeds during the years. Water waves are high in deep oceans and low near shallow shores. Wave generator performance depends on wave amplitudes which are governed by wind speeds. Water waves may become dangerous during gusts and storms. Gravity, wind and sunshine driven tidal wave and thermal power potentials were guessed earlier to be 22,000, 2000 and 87600 TWh/y [8] but recent estimates show alone wave power potential of 2985 GW [7]. Ocean temperature varies from 24 to 28 °C on surface to 4–6 °C at 1 km depths. The difference in temperature may be a basis of the ocean thermo-electric generator. Temperature differences of 20 °C are easily available in torrid and temperate zones between 30°S and 30°N latitudes [9], [8]. Tidal potential is evenly distributed worldwide, OTEC potential is high in equatorial regions, but wave energy potential is higher in the tropic zones.
Tidal power generation sites occur naturally along the sea and oceans like coal, oil, gas, shale and mineral reserves. Worldwide tidal power generation sites explored include4,200 MW Pent land Firth (UK) [10], [11] 818 to 1,320 MW Incheon and Red Tides Sihawa Bay (South Korea) [12], Kislaya Guba (Belgium) [13], 6500 MW Turnagain (USA, 2,800 MW Walcott Inlet (Australia), 5338 MW Cobequid (Canada), 7000 MW Khambat Gulf (India) and Johnstone Strait [14] and Minas Passage Bay of Fundy (Canada) [15], [16]. Some estuaries and channels, like Hudson Strait (Canada), are reported to be reminiscent of half-wave resonant oscillations [16]. Ocean thermo-electric, underwater marine and surface wave generators have relatively smaller outputs, but tidal power plants give off bulk powers. Old tidal current power stations such as 3.2 MW Jiangxia Tidal Power Station (China), 20 MW Annapolis Royal Generating Station (Canada), 240 MW Rance Tidal Power Station (France) and 250 MW Sihwa Lake power Station (South Korea) have relatively low to moderate power generation capacities, but recently planned large tidal power stations such as2,200 MW Dalupiri Blue Energy Project (Philippines), 3640 MW Tugurskaya Tidal Power Station (Russia), 8640 MW Seven Barrage (UK) and12,000 MW Mezenskaya Tidal Power Station (Russia) are large power stations.. India intends to construct a 50 MW tidal power station in the Gulf of Kutch and there is unconfirmed news of 87,100 MW Penzhinskaya Tidal Power Station in Penzhin Bay Russia [17].
There are hundreds of types of marine current turbines. The British government has started an ambitious target of 200–300 MW ocean energy by 2020. Denmark started a €3 million project for wave energy in 2012 under national initiative to produce 35% of electricity from renewable energy by 2020. As of 2012 Europe produced 246.20 MW compared to 259.20 MW by Asia. Global ocean energy production was 527.70 MW by the end of 2012 which is likely increase many times by 2020 due to multiple wave energy projects worldwide. Ireland has an estimated potential of 29 GW ocean energy [18]. Ocean thermal energy potential is much more than the tidal and wave power potentials [19]. Low temperature refrigerant based heat exchangers can to some extent extract the ocean power, but ocean thermo-electric generators (OTEG) can harvest this large natural potential if low thermal gradient high figure of merit materials become available.
Section snippets
Tidal current power
Tidal power generation depends on the rise and fall of sea and ocean waters. About 4–12 m range spring and neap tides have an estimated potential of 1–10 MW/km along the seashores. Terrestrial and celestial gravitational variations predictably affect power generation capacities. Spring tides (high tides) occur on new as well as full moons and neap tides (low tides) occur in waxing or waning half moons due to misalignment of the earth with the moon and the sun. The Earth rotates on its axis at
Wave energy converters (WECs)
Winds generate water waves on the ocean surface. These water waves are 1.3–2.86 m high near seashore several meters high in deep oceans. Water wave height depends on depth of ocean. Huge energy fluxes are available along the sea coastlines. Wind energy is converted into water waves thinly distributed in all directions over the oceans. Wave farms energy density is 2–3 kW/m2 which is higher than solar parks (0.1–0.2 kW/m2) and wind farms (0.4–0.6 kW/m2). Ocean waves have both potential and kinetic
Electrical analogy of hydraulic systems
Engineering discoveries come from mind-on and hands-on practices, simulation modeling, measurements, technical communications and professional teamwork. Innovatory engineering products take inputs from physics, chemistry, math and social sciences to make it happen. Interdisciplinary research needs mutual transformation of electrical, mechanical, hydrodynamic and thermal system models for analytical optimization studies. Analogies in electrodynamics, hydrodynamics, thermodynamics and mechanics
Ocean thermal energy converters
Ocean thermal energy may be harnessed by ocean thermal energy converters (OTEC), Ocean thermo-electric generators (OTEG) and salinity gradient osmotic power plants at points where rivers fall into the oceans. Sun shines uniformly over land masses and ocean waters. Glaciers reflect the sunlight, but oceans absorb it to heat the water. The oceans being over 70% of the earth absorb solar energy equivalent to 250 billion barrels oil equivalent. Ocean annual power production using OTEC systems is
Ocean thermo-electric generators (OTEG)
Ocean thermo-electric generators (OTEG) need usually over 100 °C temperature gradient to produce reasonable electric power. TEGs are used with incinerators to convert waste heat to electricity. They are also used on poles to convert large negative temperature difference between the ice (0 °C) and air (−50 °C) which twice than maximum ocean temperature. They can also convert concentrated solar energy into electricity [69]. Regarding ocean energy OTEGs may be used by coal power plants to convert the
Environmental impacts of ocean energy
Offshore and onshore wind farms create a noise so nobody wants to have them in their backyard (NIMBY). Ocean transport, tidal and wave power generation activities affect marine organisms [82]. Offshore wind turbines create risks of collisions, disturbance and bird migration. Electromagnetic fields and underwater noise disturb marine life [83]. When fishes pass through the turbine blades, they are injured in tidal and wave power plants. The tidal power plants change and modify wave and tide
Conclusions
Ocean energy may be harnessed by tidal power stations, wave energy converters, osmotic power plants, ocean thermal energy converters (OTEC) and ocean thermo-electric energy converters (OTEG) devices. Tidal power stations produce in the range of tens to hundreds of MW like hydel power stations, wave energy converters (WECs) from a few kW to MW, osmotic power stations from quite a few kW to MW, ocean thermal energy converters (OTECs) from kW to MW and ocean thermo-electric generators (OTEGs) from
Acknowledgements
This research was in part, supported by a grant from the Pakistan-US Science and Technology Cooperation Program (Project ID No. 299), US Department of State (jointly administered by the National Academics and Higher Education Commission of Pakistan).
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